Particulate filters of this type, more simply also referred to as soot filters, are intended to reduce the exhaust gas particulate emissions. The core piece of the particulate filter units is a substrate, in particular of SiC, which is accommodated in a separate outer housing, the exhaust pipe opening into the housing downstream and upstream of the particulate filter unit. The particulate filter insert itself, i.e. the substrate, needs to be regenerated from time to time by combustion of the particulates accumulated on the filter surface. One of the most promising possibilities of regeneration consists in introducing fuel into the exhaust gas immediately upstream of the particulate filter unit; if possible, the fuel should be in vapor form in order not to overly cool down the exhaust gas, among other reasons. This is described in EP 1 369 557 A1, for example.
Furthermore, attempts are being made to make the particulate filter units more cost-effective. The high costs of the particulate filter units are due, for one thing, to the very expensive SiC material of the substrate and to the complicated support of the substrate by means of support mats in the outer housing of the diesel particulate filter unit. As far as the incorporation into the housing is concerned, the expenditure resides, among other things, in that, because of the NO2 oxidation effect, the particulate filter units should be mounted as close to the engine as possible or are provided with heating means. The NO2 oxidation effect consists in that carbon oxidizes with the NO2 in the exhaust gas at temperatures above 230 degrees Celsius, producing nitrogen and CO2. But this regeneration effect, which is less effective in comparison with the combustion with oxygen, is to be made use of in the particulate filter unit. On the other hand, when the particulate filter unit is arranged close to the engine, extremely high requirements are to be satisfied with respect to the support of the substrate and durability of the substrate and the housing because of the strong vibrations in the section of the exhaust system close to the engine. In addition, little space is available in the exhaust system near the engine for the voluminous particulate filter units. The consequence is a number of factors which are partly contradictory and, when one first looks at it, appear to be mutually exclusive as regards the design and arrangement of the particulate filter unit with a view to a more cost-effective configuration.
Concepts of accommodating a main filter and a pre-filter in the particulate filter unit itself, i.e. in the same outer housing, have been disclosed in U.S. Pat. No. 5,053,062 and EP 1 205 228 A1 and also in EP 0 957 241 A1. But these concepts do not provide for a truly marked reduction in the costs of a particulate filter unit or for a distinctly simplified accommodation of the exhaust system in the vehicle.